Electronic counter



9 Sheets-Sheet l N. F. MOODY ETAL ELECTRONIC COUNTER Oct. 5, 1954 FiledJuly 24, 195o Oct. 5, 1954 N. F. MOODY ETAL ELECTRONIC COUNTER 9Sheets-Sheet 2 Filed July 24. 1950 N. F. MOODY ETAL 2,691,100

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ATTORNEYS- Patented Cet. 5, 1954 UNI STTS RTNT OFFICE ELECTRNIC COUNTERApplication `l'uly 24, 1950, Serial No. 175,514

Claims priority, application Canada. August 4, 1949 (Cl. 25d-27 2Claims.

The invention relates to electronic counters 0f a type suitable forautomatic counting of rapidly occurring events such as cosmic rays,radiations from radio active material or volla age transients in anelectric circuit.

A commonly used electronic counter circuit is a series chain of binaryswitching circuits have ing trigger pulse feed back connections betweencertain of the switching circuits. Each binary switching circuit,sometimes called a multivibrator circuit or a flip-flop circuit,comprises pair of triode or pentode tubes having an in terconnectionbetween the anode to cathode circuit of each tube and the control gridcircuit of the other tube so arranged that conduction through one of thetubes will stop conduction in the other tube. The anode potentials ofeach tube change from a low value during periods of conduction to a highvalue during periods ci non-conduction, and the circuit is so arrangedthat an input voltage pulse applied to the interconnections in parallel,causes the state of con duction of each tube of the pair to be changed.Each switch from a state of conduction to a state of non-conduction ineach tube produces a sudden rise of anode potential of that tube, andeach switch from the state of non-conduction to the state of conductionproduces a sudden iall of anode potential. The rise and fall in anodepotential form the edges of a square wave voltage pulse and, if only thesquare wave volt age pulses produced by one oi the tubes of the pair areapplied to the input of the next switching circuit, the result is asingle input pulse to the next switching circuit for each two inputpulses to the 1st switching circuit.

The natural counting scale of a series chain of four binary switchingcircuits is sixteen since. for each sixteen pulses applied to the inputof the ist switching circuit, only one pulse is produced in the outputcircuit of the 4th switching circuit due to each of the switchingcircuits having the effect of dividing by two the number of pulsesreceived by it. For each sixteen input pulses fed to the lst binaryswitching circuit, 8 pulses are fed to the 2nd binary switching circuitwhich in turn divides by two producing four pulses in the input oi the3rd binary switching circuit. The 3rd binary switching circuit dividesby two producing two pulses in the input of the l lith binary switchingcircuit which produces the single output pulse of the chain.

For rnost applications it is much more convenient to have a countingscale of ten (decade scale) as required by the decimal system instead ofthe natural counting scale of sixteen, and usually the series chain offour switching circuits is modified to obtain an artificial scale or tenby having trigger pulse feed back connectons between certain of theswitching circuits. An artificial scale of ten may be obtained by havingtrigger pulse feed back connections between the 3rd and 2nd switchingcircuits and between the 4th and 3rd switching circuits. Each time the2nd switching circuit produces a pulse at the input of the 3rd switchingcircuit (the rst four pulses of the decade scale having been applied tothe lst switching circuit), the 3rd switching circuit feeds back atrigger pulse to the 2nd switching circuit causing the 2nd switchingcircuit to be immediately reset so that the state of conduction of thetubes in the 2nd switching circuit is the same as that corresponding tosix pulses having been applied to the 1st switching circuit of the chainalthough actually only four pulses had been applied. Similarly,- uponthe third switching circuit transmitting a pulse to the fourth switchingcircuit, the fourth switching circuit feeds back a trigger pulse to thethird switching circuit causing the third switching circuit to beimmediately reset so that thev state of conduction of the tubes in thethird switching circuit is the same as that corresponding to twelvepulses having been applied to the first switching circuit of the chainalthough actually only six pulseshad been applied. The nextv four pulsesapplied to the iirst switching circuit of the chain cause the rstswitching circuit to. apply two pulses to the second switching circuitwhich, in turn, divides by two applying a single pulse to the thirdswitching circuit. The third switching circuit then applies a secondpulse to the fourth. switching circuit causing it to supply.

a single pulse in the output of the chain.

Continuous indication of the kprogression of the count in a chain ofswitching circuits can be obtained by combining voltages obtained fromthe anode potentials of the tubes in the switching circuits. Asexplained above, the anode potential of each tube in the chain is eitherhigh or low depending on how many pulses have been applied to the inputof the chain andy with the anode potentials as voltage sources a uniquelamp of a series of cold cathode gaseous discharge lamps can be litduring each count of the scale. By placing several decade count chainsof binary switching circuits in series, a counter be. formed capable ofcounting to a high number. The first chain of the series would countunits, the second hundreds, the third thousands, etc. For example, vedecade count chains in series could register a maximum count of 99,999.

Prior to the invention, electronic counters have not been sufficientlyreliable for some applications, such as the counting of radiations fromradio active material when safety depended on the accuracy of the count.There have been several reasons for the lack of reliability, and one ofthe most important of these is the variation in tube parameters withageing. Since indication is provided by discharge lamps having ratherclose striking and extinguishing voltages, for example, a strikingvoltage of '75 volts and an extinguishing voltage of 55 volts, it isimportant that the anode potentials of the tubes in the switchingcircuits during their state of conduction and non-conduction be atuniform levels. In counters, according to the prior art, it oftenhappened that the low level of the anode potentials of the tubes in theswitching circuits would vary by as much as 25 or more volts as a resultof tube ageing which is accompanied by loss of emission. This change inthe anode potentials affected the voltages applied to the dischargelamps so that often a lamp which should be extinguished would strike, orvice versa, giving an inaccurate count.

Another source of lack of reliability in previously known electroniccounters, was the number of ways in which voltages obtained from theanode potentials of the tubes in the switching circuits could becombined to produce the striking and extinguishing voltages for thedischarge lamps. In a decade chain, the common practice is to group theten lamps into two groups of five, one electrode of each lamp in eachgroup being connected together and the common connection of theelectrodes in each group being connected to the anode circuits of thefirst switching circuit in the chain so that one group was supplied witha voltage on even number counts and the other group was supplied with asimilar voltage on odd number counts. Certain common connections weremade between the other electrodes of the lamps and these commonconnections were connected to appropriate anode circuits in the 2nd, 3rdand 4th switching circuits with the result that during a counting cyclethere were a great many combinations by which the striking f andextinguishing voltage could be obtained.

Still another' source of unreliability was the feed-back circuits usedbetween the switching circuits for reducing the natural scale count ofsixteen to an articial scale count of ten. The feed-back circuits shouldbe disconnected from the pairs which they retrip at all times exceptwhen actual retrip occurs. It will be evident that if this is not so theelements associated with the feed-back circuit impose a load on the gridof a tube of the previous pair and so reduce reliability.

In the circuits of previously known electronic counters, the inputpulses to each switching circuit were fed through a shaping circuit tothe interconnections between the anode to cathode circuits of one tubeand the control grid circuit of the other tube. A shaping circuitaccording to the prior art consisted of a resistor-capacitor network inwhich the capacitor was series connected from the source of square wavepulses to the interconnections between the tubes of a switching circuit,and the resistor was series connected from the source of anode voltageto the interconnections. The resistor-capacitor network shaped eachsquare Wave pulse into a sharp positive pulse corresponding to therising edge of the square wave pulse and a sharp negative pulsecorresponding to the falling edge of the square wave pulse. Whichevertube of the switching circuit happened to be conducting at the time thesharp positive pulse occurred, would adsorb the positive pulse and thesharp negative pulse would then cause the states of conduction of thetubes to be switched. However', often it was possible for the positivevpulse itself to be differentiated in the networks of the binary pair towhich it was applied. Ii so, the falling edge of the positive pulseproduced a negative pulse which was only too likely to trip theswitching circuit erroneously. A further disadvantage of such a systemwas that the anode current for the tubes passed through the resistor ofthe shaping circuit and the voltage drop which resulted made itnecessary to use a source of higher anode Voltage than would otherwisebe required.

An electronic counter according to the invention overcomes thedisadvantages of the prior art and is both reliable and accurate in itsoperation. According to the invention, the tubes in the switchingcircuits are operated in a bottomed condition resulting in very accuratedetermination of the low level of anode potential, during the periods ofconduction of the tubes. In this speciiication, and in the attachedclaims, a tube is said to be bottomed when its load line intersects theline of coalescence of the characteristic anode current vs. anodepotential curves of the tube. In the case of a bottomed tube, the anodepotential ceases to fall as soon as the control grid becomes lessnegative than the point of intersection of the load line and thecoalescent characteristic curves. Since the coalescent portions of thecharacteristic curves are almost parallel to the anode current ordinate,a fall in the emission in the tube due to ageing will cause only a veryslight rise in the level or the anode potential when the tube is in aconducting state. In addition, the low level of anode potential dependson the highly stable properties of the tube in the case of a bottomedtube and so the low level is not aiected by any variation which mightoccur in the supply lines, resistance tolerances and tube tolerances.

According to the invention, the indicator lamps are grouped into evennumber indicator lamps and odd number indicator lamps by commonconnections to the 1st switching circuit in a manner similar to theprior art practices except that each connection to the 1st switchingcircuit is through a rectifier. Use of rectiers, which transmit avoltage of a given polarity only, reduces the number of possiblecombinations of voltages which might occur and cause improper operationof the indicator lamps.

In an electronic counter, according to the invention, feed back from oneswitching circuit t0 another, is through a rectifier. A second rectiileris used to limit the bias of the non-conducting tube of the switchingcircuit to which a pulse is fed back so that the tube is able quickly toswitch from its non-conducting to a conducting state. Use of the secondrectiner shortens the length of time required for switching in theswitching circuit to such an extent that there is no danger of the nextswitching circuit being triggered at the same time, and also shortensthe minimum time interval which must separate two pulses so that theycan be counted independently (i. e. improves the resolving time).

In a shaping circuit according to the invention, an inductance and arectifier are placed in shunt with the resistor which is seriesconnected between the source of anode voltage and the interconnectionsof the tubes in the switching circuit with the result that the lowdirect current resistance of the inductance substantially eliminates thedrop in voltage due to the shaping circuit and, consequently, a lowersource of anode voltage can be used. However, the inductance presents ahigh impedance to the square wave pulses and so does not aiect theoperation of the circuit in respect to pulses. The rectifier isconnected so that it eliminates the pulses corresponding to one of theedges of each input square wave pulse. Use of the shunt inductanceprevents the rectifier from being biased by the voltage drop that wouldotherwise form across the resistor.

The invention will be further described by reference to the attacheddrawings which illustrate certain embodiments of it, and in whichFigures 1(51.) and 1dr), (subsequently referred to as Figure 1) togetherform a schematic diagram of an electronic counter according to theinvention.

Figure 2 is a block diagram corresponding to Figure 1,

Figure 3 is a simplified schematic diagram of part of Figure l showing abinary switching circuit, according to the invention,

Figure 4 shows oscillograms for the binary switching circuit shown inFigure 3,

Figure 5 is a chart comparing natural count to decade count withindication of conduction or non-conduction in the switching circuittubes shown in Figure 1,

Figure 6 is a graph of the operating characteristics of a tube as usedin a binary switching circuit according to the invention.

Figure is a simpliiied schematic diagram of part of the neon lampcircuit of Figure 1,

Figure S is a simplied schematic diagram of the circuit for neon lampsNo. 0 and No. 1,

Figure 9 is a chart showing the cyclic conditions of neon lamps No. 0and Figures 10m) and 10(1)) (subsequently referred to as Figure 10)together show oscillograms for the schematic diagram shown in Figure 1.

A schematic circuit diagram for a series chain of binary switchingcircuits having gaseous discharge indicator lamps is shown in Figure 1of the drawings. As shown in Figure 1, and in block form in Figure 2,the series chain comprises four biliary switching circuits, the 1sthaving tubes Vi and V3, the 2nd Vli and V, the 3rd Vl and V3 and the 4thVic and Vi I. Some of the parts in each switching circuit fulfillsimilar purposes and are identical in electrical characteristics, and inthe following description of the circuit, the same designation will beused for such a part each time it appears in the circuit. The values ofthe resistors and capacitors used in the circuit are indicated in Figure1 and, where it is necessary by way of explanation, they will bereferred to in the following description. However, the electrical valuesand ratings of the components used are matters of design and can bereadily calculated by those skilled in the art.

In each switching circuit there are interconnections 20, eachinter-connection being from the anode 2! of one tube through a resistorand a capacitor in shunt to the control grid 22 of the other tube. Inthe 1st switching circuit the resistor is designated as Ri having avalue of 100,000 ohms (K) and in the 2nd, 3rd and 4th switching circuitsthe resistor is designated R2! (330K). The capacitor in shunt to theresistor RI and the capacitor in shunt with the resistor R2! in the 2ndswitching circuit is designated as C! having a value of 30 ,cpi and inthe 3rd and 4th switching circuits the capacitor in shunt with theresistor R2! is designated as C2 having a value of 50 caf. The anodes 2|of the tubes Vl and V3 are connected to the input connection 23 for the1st switching circuit by a resistor R2 (20K) and the anodes 2i of thetubes in the other switching circuits are each connected to itsrespective input connection 23 by a resistor R22 (33K). The inputconnection 23 for the 1st switching circuit has a series capacitor C3(100 ccf.) and in the case of the 2nd, 3rd and 4th switching circuits,the series capacity in each of the input connections 23 is Cil (200auf.) In the case of the 3rd switching circuit, there is a resistor R20(390K) from each of the anodes 2| of the tubes V1 and V9 to ground. Theinput connections 23 to the 1st switching circuit is connected to a +100volt supply connection 24 by a resistor R3 (10K) in shunt with aninductance Ll and a rectier Xi which may be, for example, a crystal ordiode rectier. The input connections 23 to the 2nd, 3rd and 4thswitching circuits are each connected by a resistor R3, an inductance Lland a rectier Xl to a volt supply connection 25 in the same manner asthe input connection 23 of the 1st switching circuit is connected to the+100 volt connection 24.

The voltage supply for the screen grids 26 of each of the tubes in the1st, 2nd and 4th is from the +100 volt connection 24 through a resistorR4 (18K) and the voltage supply for the screen grids 26 of each of thetubes in the 3rd switching circuit is from the +150 volt connection 25through a resistor R5 (27K). There is a capacitor C5 (200 cui.)connected between the screen grid 2S oi each of the tubes Vl, V4, V1 andVIE! and a common ground connection 2l. Control grid bias to each of thetubes in the switching circuits is supplied from a 100 volt supplyconnection 2B through a resistor, R0 (200K) in the case of the 1stswitching circuit and R23 (390K) in the case of the 2nd, 3rd and 4thswitching circuits, The resistor R6 for the tube V3 and the resistor R23for each of the tubes, V5, VS and Vii connect to a -100 volt resetconnection 29 which is connected to the connection 23 through apush-to-break switch 30.

As shown in Figure 1, there is a cold cathode gaseous discharge lamp foreach count of the decade scale, and each lamp is designated by the countnumber which it indicates. A common connection 3i connects together oneelectrode of each of the lamps which indicate an even number, No. 0, No.2, No. 4, No. 6 and No. 8, and a common connection 32 connects togetherone electrode of each of the lamps which indicate an odd number, No. 1,No. 3, No. 5, No. 7 and No. 9. rihe remaining electrodes of the lampsare connected together in pairs by a common connection 33 between No. 0and No. 1 lamps, a common connection 34 between No. 2 and No. 3 lamps, acommon connection 35 between No. 4 and No. 5 lamps, a common connection36 between No. 6 and No. 7 lamps and a common connection 31 between No.8 and No. 9 lamps. The common connection 32 between the odd number lampsis connected to an anode 38 of a double diode rectier tube V2 and thecathode 30 corresponding to the anode 38 is connected through a resistorR1 (100K) to the anodejZI -of the tube VI. The common connection 3|between the even number lamps is connected to an anode 40 of the doublediode V2 and the cathode 4| corresponding to the anode 40 is connectedthrough a resistor R1 (100K) to the anode 2| of the tube V3. The lament42 of the tube V2 has one side connected to ground through a resistor R8(220K) and connected to the +100 volt connection 24 through a resistor R(220K). A capacitor C5 (.05 afd.) is in shunt with the resistor RB.

The connection 33 between No. 0 and No. 1 lamps is connected by aresistor RIU (200K) to the anode 2| of the tube V4 and through aresistor RI 0 to the common connection 36 of the lamps No. 6 and No. 7.Another resistor RIG from the common connection 33 connects to the anode2| of the tube VID, and through a resistor RII), to the commonconnection 35 of the lamps No. 4 and No. 5. A resistor RIO connects thecommon connection 34 of the lamps No. 2 and No. 3 to the anode 2| of thetube V5 and, through a resistor RID to the common connection 31 of lampsNo. 8 and No. 9. Another resistor RIE? from the common connection 34connects to the anode 2| of the tube V1. 'Ihe common connections 35 oflamps No. 4 and No. 5, and 31 of lamps No. 8 and No. 9, each connectthrough a resistor RII) to the anode 2| of each of the tubes V9, andVII, respectively. The common connection 35 of lamps No. 6 and No. 7connects through a resistor RI to the anode 2| of VI I.

The feedback connection between the 3rd and 2nd switching circuits isthrough a series capacitor V1 l00 Mii.) between the screen grid 2B ofthe tube V1 and a cathode 43 of a double diode rectifier tube V5. Theanode 44 corresponding to the cathode 43 is connected to the controlgrid 22 of the tube V6 and through a resistor RII (270K) to the groundconnection 21. The control grid 22 of the tube V4 is also connected tothe ground connection 21 through a resistor RII. The feedback connectionbetween the 4th and 3rd switching circuits is through a series capacitorC3 (150 fuif.) between the screen grid 26 of the tube VI and a cathode45 of a double diode rectifier tube VB. The anode 46 corresponding tothe cathode 45 is connected to the control grid 22 of the tube V9.Positive bias is supplied to the cathodes 43 and 45 by a voltage dividercomprising the resistors RI2 (120K) and RI 3 (30K) between the +150 voltconnection 25 and the ground connection 21. The connection between theresistors R|2 and RI3 is bypassed to the ground connection 21 through acapacitor C9 (.05 pid.) and is connected by a connection 41 to a commonconnection between resistors RM (100K.) and RI5 (50K) which are inseries between the cathode 45 of the tube V8 and the cathode 43 of thetube V5.

The control grid 22 of the tube V4 has a bias connection by the cathode48 and its corresponding anode 49 of the tube V5 to the commonconnection between a pair of voltage dividing resistors RIG (850 ohms)and R|1 (1.7K) between the ground connection 21 and the 100 voltconnection 28. A bypass capacitor CIO (.01 afd.) is connected from thecommon connection of the resistors RIS and R|1 to the ground connection21. The anode 49 is connected to the anode 50 of the tube V8 and thecathode 5I corresponding to the anode 50 connects to the control -grid22 of the tube V1.

The +100 volt connection 24, the +150 volt connection 25, the kgroundconnection 21 4and the volt `connection 28 are Vsupplied with theirrespective voltages from any suitable, well regulated supply of D. C.voltage. As shown in Figure 1, there is a bypass capacitor CII (.1 afd.)from the +100 volt connection 24 to ground, and a resistor R|8 (4.2K)between the +100 volt connection 24 and the +150 volt lconnection 25.The -100 volt connection 28 is bypassed to the ground connection 21through a capacitor CI2 (.01 pid).

The operation of a binary switching circuit is characterized by the factthat it has only two stable states: either one tube will conduct and theother will be cut off or vice versa. In the simplified circuit for the4th switching circuit shown in Figure 3, transition from one state tothe other occurs when a negative voltage pulse is applied to the inputconnection through the capacitor C4. If the switch 30 is opened, thenegative bias from the -100 volt connection 28 will be removed from thetube VI I which must therefore conduct, so that the tube VII) will becut off by the negative bias applied to its control grid 22 via theresistors R2| and R23. 'Ihis state will be maintained when the switch 30is reclosed, and will be the starting point for the explanation of theoperation of the circuit.

With VII) cut off the anode 2| of VII) will rise in potential to aboutvolts, the diierence of 25 volts between that value and the 150 voltpotential of the volt connection 25 being caused by the loads RZI andthe indicating lamp circuits (not shown in Figure 3). The tube VII willdraw control grid current whose value is given by t R21 R23 330K 390Kand so the control grid potential of VII is substantially at earth.

The curves shown in Figure 6 indicate the condition in the tube VII(with the screen grid 2B at +30 volts when the control grid 22 is at 0volts) such that the intersection of the load line for R22 (33K) gives avalue of EA (anode potential) of 10-15 volts. Since the load lineintersects the curve below the knee of the pentode characteristic curve,the condition is known as bottoming and the low level of the anodepotential is very accurately set in a manner which is reasonableindependent of variations in tube parameters for a given tube. In thisconnection it is seen that even a negative bias of -1.5 volts applied tothe grid will raise the anode potential only by about 10 volts whichwill not disturb the operation, and corresponds to the reduction ofcathode current which may be expected with a tube during its life.

The condition of bottoming occurs if the cathode current is set to avalue which is about twice the anode current instead of the usual 1.1 to1.3 associated with pentodes. For a iixed anode load, the anode currentcannot exceed the intersection of the load line with the ordinate(Figure 6), so that the surplus cathode current must flow to the screen.By correct choice of screen voltage any desired cathode current can beset up at a particular grid bias, in this case zero bias. However, thescreen voltage required may vary from tube to tube, but should thescreen be fed via a resistor from a source voltage large compared tothat required to supply screen potential, this resistor will itselfcontrol the screen current and the cathode current itself will belargely independent of the tube characteristics. For

'9 the circuit in Figure 3, the potential of the screen grid 26 of VI Iis 30 volts obtainedfrom the +100 volt connection via R4. Should thetube age, as mentioned above, the screen current will fall far more thanthe anode current, (for the example quoted, -1.5 volts reduced the anodecurrent by about 0.6 ma. but the cathode -current was reduced by 3 ma),and it follows that the screen grid potential will rise so as largelytocompensate for the loss of cathode emission. Thus screen voltage ratherthan anode voltage is a measure of tube ageing, and the method ofoperation employed gives exceptionally long life since tubes mustdeteriorate far before they fail.

The operation of the circuit when pulses are received will now bedescribed. Imagine an input square wave with steep edges and 75 voltampitude applied to Clt in Figure 3, then the square Wave will appear onthe connection 23 in differentiated form. The waveforms of thisdiscussion are shown in Figure 4 where A and B are the input anddifferentiated Waves. When the square wave has a positive edge C4 willbe charged by the rectifier XI without producing any significant voltageon the connection 23, but on the negative edge of the voltage onconnection 23 will fail and C4 Will be recharged through R3 giving atime constant of recovery of 2 `microseconds. Inductance LI plays nopart in this action since its inductance is relatively great. InductanceLI serves to pass the anode currents of VII) and VII without causing therectifier to be permanently biased, as would happen ywere it omitted.

A 50volt negative pulse appears on connection 23, 25 volts being lost inthe shaping process, and this wave finds its way to the anodes 2| viathe resistors R22. Assuming VII to be conducting, depressing connection23 by 50volt pulse temporarily reduce its anode current. However, sincethe tube is bottomed, the change of current will not iniiuence itspotential signicantly, so that the effect of the pulse at this point maybe ignored. With VIII, however, it is another matter. Since the valve iscut off, the anode voltage will fall as the potential of the connection23 is depressed, and by a somewhat smaller amount. This fall will betransmitted to the control grid 22 of VI I via C2 thus cutting off anodecurrent in VII.

Current will be stopped in VI I in about 0.1-0.2 microsecond after theinstant of triggering and the anode will commence to rise after thisdelay time. It must rise about 40 volts to overcome the bias on thecontrol grid 22 of VIO, the rise being transferred with little loss viaC2, and this may be expected to occupy another 0.2 microsecond.

In 0.3 to 0.4 microsecond, that is within the trigger pulse duration (2vmicroseconds), the changeover of current has taken place. Thereafter asthe control grid 22 of VII) is switched on, its anode falls rapidly, sothat the control grid 22 of VII is taken to about -90 volts, from whichis slowly recovers (time constant microseconds) to its quiescent valueof -40 volts and control grid current flows in VIII.

The rising anode voltage of VII completes the first 40-50 volts of itsexcursion charging stray capacities only: once control grid currentflows in VIO, C2 must be charged and the recovery is completed with atime constant of about 2 microseconds. In this connection it should benoted that the voltage of the connection 2'3 Was depressed 50 volts bythe pulse so 10 that an alternative limit )is set v.to the .rise of thev'anode potential of VII by the -decay of the trigger pulse.

A second pulse arriving willlretr-ip the pair in the opposite direction.It may be wondered why subsequent pulses .reverse the -state Vofconduction since the circuit is symmetrical. This results particularlyfrom the storage lof charges in the capacitors C2 -corresponding to the'differing control grid potential before the pulse is applied. Thischarge is not greatly altered during the trip action onfeithercapaci-tor, fand is maintained or remembered for 10 microseconds aftertripping 'at the grid which -is `being driven negatively (VII in abovevdescription-- see also waveform E, Figure 4). The capacitors C?.therefore constitute a memory, which control the system during thetrigger pulse, and .is maintained for a few microseconds thereafter.During the recovery or storage of the new charge condition the circuitwill refuse to retrip or will need a larger pulse `to cause thetransition.

Due to these effects one might expect the resolving time of the lastpair to be about 10 microseconds, and this is roughly true. Subsidiaryfeatures of note exist. When anode 'current in either vtube is cut off,'the screen voltage rises to 100 volts and evidently 4the 'cathodecurrent vof that valve is then potentially high. When this tube isswitched on, the cathode'current is therefore several times its nalvalue for the microsecond or so needed to discharge the screencapacities. This explains the rapid fall of 'theanode voltage of VIII asshown in Figure 4 bycurve C and mentioned above. During this fall theanode will bottom rapidly, and once in this condition, the anode currentis limited to 4 ma. so that perhaps 10 ma. is .available to the screengrid.

A Ascreen grid is used to generate fthe square waves for triggeringsubsequent pairs, Vand the initial rapid fall is very necessary toperform this function satisfactorily. In the pair -described, capacityis not shown on the screen grid of VI I, but it is contributed by theshaping circuit of the first switching circuit of the ynext decade (C3,R3, LI and XI). Whilst the screen potential is rising, C3 will chargevia XI, but on the negative edge where the next pair is to trigger, onlythe `stray capacities lneed charging so that a fast negative edge isgiven.

The third Aswitching circuit which includes Vl and V9 does not diifer inprinciple from the 4th switching circuit described above, but `includesin its circuit the double diode V8 and has the value of its screenresistors raised from 18K to 27K with connection to the -I-150-voltconnection 25 instead of the -I-IOO-volt connection 24. These changesare associated with the mechanism for producing the decade count andwill be discussed later in connection with its operation.

The only basic differences between the circuit of the 2nd switchingcircuit and that of the 4th switching circuit is that in the 2nd thereis a double diode V5 used for the reasons described above in connectionwith double diode V8, vas well as a reduction in control grid timeconstant due to the coupling capacitors CI being 30 paf. and theadditional shunt resistors RII (270K)., connected from the control grid22 to ground. These changes reduce the control grid time constants from10 microseconds to about 4 microseconds, and limit the grid excursion ina negative direction. The former is the more sigl l nicant factor inreducing resolving time, but the reduction of grid bias on the cut-olitube does facilitate retriggering before full recovery of the gridmemory circuit and thus provides a safety factor in the reliability ofoperation.

The resolving time of the lst switching circuit is a limiting factor forthe resolving time of the complete series chain of the four switchingcircuits. For a 2.5 microseconds resolving time in the 1st switchingcircuit, the 2nd switching circuit may have a resolving time of 5microseconds so that it is the lst switching circuit which presents thegreater difficulties.

Limits to the resolving time are set by:

To a limited extent a heavy drive pulse or increased triggeringsensitivity is helpful in reducing resolving time, but no more than 50%can be gained by this means, and such practice should be used only togive additional safety.

Considering these requirements in turn the ist switching circuit ismodied by comparison with the 4th switching circuit as follows:

(i) Trigger pulse reduced to l microsecond (03:100 Mii).

(ii) Grid time constant 2 microseconds approximately (C|=30 paf.,coupling resistors Rl and R6=100K and 200K respectively).

(iii) Anode time constant reduced to about 0.6-

0.8 microsecond by use of 30 auf. coupling condenser-s and 20K anodeloads R2.

To allow for the reduced anode loads without excessive anode current,the anode supply is taken from the +100 volt connection 24 instead ofthe +150 volt connection 25 as in all other switching circuits.

It is a great aid in counting operations to have a decimal system ofscales. The benefit is furif: its screen grid falls, the 3rd switchingcircuit will switch so that the tube Vl conducts. Anode configurationsare shown up to this stage in Figure 5, decade count, where the laststate is shown as count No. 4A. So far the operation has not differedfrom that of a series chain of switching circuits without feedback.

However, state 4A is maintained for only a fraction of a microsecond. Asthe screen grid of the tube V'Z falls a negative impulse is applied viaCl and the right hand portion of the double diode tube V5 to the controlgrid of the tube V6. The latter tube was conducting as a result of thefourth pulse, but is now cut off so that the 2nd switching circuitswitches again, automatically. The effect, therefore, is that the 2ndswitching circuit merely transmits a triggering pulse to the 3rdswitching circuit while the iinal state of the 2nd switching circuit isundisturbed and the arrangement of conduction is now as shown at countLl, decade count, of Figure 5.

On the arrival of the 5th pulse the 2nd switching circuit switchesagain, the tube V 6 conducts once more and results in a switch of thestates of conduction in the 3rd switching circuit. The tube V9, whichhad been cut oii, now draws current and its screen grid falls thustriggering the 4th switching circuit. This state, which is transitory,is shown in Figure 5, decade count, as 6A.

By similar mechanism the fall of the screen grid of the tube Vl appliesa negative pulse via the capacitor CS and the right hand side of doublediode tube V8, so that after a fraction of a microsecond, the tube V9cuts olf and the 3rd switching circuit reverts to the original states ofconduction in its tubes. The anode quiescent conditions are shown ascount 6 of Figure 5, decade count. rThe effect is that the 3rd switchingcircuit has transmitted a pulse to the fourth switching circuit, but thefinal state of the 3rd switching circuit is undisturbed.

Since the 3rd switching circuit has been triggered twice in thisactiononce by pulse 6 and once by the feedback, it may be wondered whythe 2nd switching circuit has not been disturbed by ther increased, andcorrect operation is more readily checked, if each of the ten counts isindicated by a single neon so that no addition is required ininterpolation. The 1st switching circuit divides by two and, if thedecimal system is to be followed, the remaining switching circuits (anatural scale of 8) must divide by 5. Since the 1st switching circuitwill deliver pulses to the divide by 5 sections only for each alternateinput pulse, the divide by 5 section may be considered independently asreceiving pulses at 2, 4, 6, 8, and l0 Vcounts respectively, and it willbe necessary only to consider these pulses to explain its operation. Thechart shown in Figure 5 gives a comparison between the configurations ofthe states of conduction in the tubes of the switching circuits for eachcount in both the natural and artificial scales. As shown in Figure 5,with the counter set to zero by the reset button, the right hand tubesV3, V6, V9 and VI I will all be conducting. On the second pulse VG inthe 2nd switching circuit will cut oi, but this will not inuence the 3rdswitching circuit, since the positive edge at its screen is absorbed byXl after passing C0. The arrival of a fourth pulse will render the tubeV 5 conducting again, and since the feedback loop between the 3rd and2nd switching circuits. This point will be discussed later, and at thisstage it suffices to say that the 3rd switching circuit must remain withthe tube V1 out ofi for about 2 microseconds before the retrip circuitis primed, and the retrip action between the 3rd and 4th switchingcircuits is far shorter than this.

The remainder of the counting cycle is entirely normal as is seen bycomparing the last four counts of the decade count with the last fourcounts of the natural count in Figure 5. The 3rd and lith switchingcircuits are not tripped again so that the feedback plays no part in theremainder of the counting cycle. Since all the unusual actions takeplace between the counts 4A and 4 and between 6A and 6, furtherconsideration will be given these counts in the following explanation ofthe feedback action.

Feedback pulses from the 3rd to the 2nd switching circuit are taken fromthe screen grid of the tube Vl and are applied via the double diode tubeV5 to the control grid of the tube V6. The screen grid and diode cathodewaveforms are shown in Figure 10, F and G respectively.

Referring to the screen grid waveform F, the significant features arethat there is only one large negative edge at the count of 4, and thatthe screen rise at counts of 0 and 10 is governed mainly by a 6microsecond time constant due to C and R5. The Waveform F isdifferentiated by C'l and RIS (whose time constant is substantially thesame as that of C5 and R5) so that the Waveform appearing at the cathode43 of the diode tube V5 is, as shown in Figure 10, G, in which thedotted line, the conduction level of the diode tube V5, conductionoccurring at voltages negative to this level.

On the fourth pulse, the screen grid voltage of the tube V1 falls verysharply and substantially 85% of the fall (60 volts) appears at thediode tube V5 and 3i) volts is transmitted by the diode tube V5 to thecontrol grid of the tube V6 which, at that time, is conducting. The 2ndswitching circuit quickly retrips, and this is facilitated by the factthat the tube V11 cannot be biased by more than 33 volts negative byvirtue of the left hand portion of diode tube V5 being at 33 voltsnegative. Without the diode tube V5 this grid would be at about 90 voltsnegative since the switching` circuit has just tripped, and resettingwould be more unreliable. On the 6th pulse, as explained above, the 3rdswitching circuit is tripped and rapidly reset by a feedback pulse. Thisaction probably takes around 1 microsecond, so that the screen grid ofthe tube V7 rises in voltage by only a small fraction of its completeexcursion due to the 6 microseconds time constant. Furthermore thediffering time constant oi C? and Ri5 (5 microseconds) does notdifferentiate appreciably and is so short a time that there isnegligible swing in a negative direction `ee the 3rd waveform of Figure10, G). The small negative swing` is far below the threshold valueneeded to overcome the bias of the section of the diode tube V5 inseries with the feedback circuit.

Turning now to the feedback circuit between the 3rd and 4th switchingcircuits, the voltage of the screen grid of the tube VID rises on the thpulse (also denoted 0) and falls on the 6th pulse. After diierentiationby the network C8 and Ri (10 microseconds time constant) the waveformsare as shown in Figure 10, K, and resetting of the 3rd switching circuitoccurrs in a manner similar to that described above in connection withresetting of the 2nd switching circuit. A complete set of waveforms forall the switching circuits is given in Figure 10 and these are selfexplanatory.

In the indicating lamp circuit, as shown in Figure 1, the neon lamps aregrouped into an odd and an even bank and the common connection 32 fromthose in the odd bank is made via the diode tube V2 to the anode 2| ofthe tube Vl and the even bank is similarly connected to the anode 2i ofthe tube V3 by the common connection 3i. The free connection of eachneon then connects via two resistors Ri, to two anodes other than thosein the lst switching circuit. The cathodes 39 and fil of the diodesfeeding the odd-even lines 32 and 3i, can have either of two potentials:10 volts or 80 Volts. The free side of each neon lamp can take up themean potential of the two anodes to which it is connected, and theanodes can be at 125 volts or 10 volts. Since both anodes can be at 125volts, or one at 125 volts and one at 10 volts or both at 10 volts it isseen that three combinations of voltage are set in this way. Two furthercombinations are set by the odd-even selection of the rst switchingcircuit making 6 combinations in all. Since no more than 6 combinationscan occur, only 6 neon lamps need to be considered, and the voltagestresses applied to these calculated in order to ensure that one onlywill illuminate for a single count. A further simplification may be madeby replacing the two resistors RIE! with a single resistor R23 (Figure7) of half the value having the mean of the two anode voltages impressedon it. Figure 7 shows the simplied circuit, and the stresses applied tothe neon lamps cover all cases existing in the lcounter circuit ofFigure 1. The numbering of the neon lamps in Figure 7 is not intended toagree with the number-ing of the neon lamps in Figure l, but is used toindicate odd and even lamps.

Nominal values or" operating voltages for neon lamps will be taken asfollows for purposes of explaining the circuit: striking voltage 75volts, burning voltage 60 volts and extinguishing voltage 55 volts. Iiit be assumed that No. 1 neon lamp only can light, then the stressacross it is (125-10) volts=115 volts and since this exceeds thestriking voltage of '75 volts it will strike, and thereafter burn at 50volts.

The current flowing is (-60) volts 200K ohms Evidently the No. 3 and No.5 neon lamps are stressed by the potential at the anode 38 relative tothat on the ends of the series resistors R23. This potential is1-l(100,000 275 lia.) :37.5 volts. Then the stress on No. 3 neon lamp isS75-375:35 volts and, since this is less than 55 volts (the extinctionpotential), the neon cannot be alight. Note that should the No. 1 neonlamp fail to light, the potential. of the anode 3i) would be 10 voltsand the No. 3 neon lamp would be stressed to 57.5 volts and mightstrike. In this unlikely event a negligible current would now so thatNo. 1 neon lamp would remain fully stressed, and when it strikes itwould extinguish No. 3 neon lamp.

No. 5 neon lamp is stressed with (37.5---10 volts)=27.5 volts and socannot light.

Turning now to the even numbered lamps, No. 2 lamp has the stressexisting between the point A (Figure 7) and the 80 volt potential of thediode cathode d2. Since the point A is 27.5 volts below volts (due tothe 275 ta. drawn drawn by No. 1 neon lamp) the stressis (97.5-80)volts=15 volts so that it cannot light. If the No. l neon lamp weredefective and failed to operate the stress on the No. 2 neon lamp wouldbe higher, namely 45 volts, and should it strike at this voltage point Awould be at (804-45) vo1ts=l25 volts so that No. 1 neon lamp has thefull voltage applied to it and when it does strike, it will extinguishNo. 2 neon lamp. No. 4 neon lamp cannot light by its direct path, sinceits 15 volt stress is in any case in the wrong direction for the diodeV2 in the common line 3| to conduct.

No. 6 neon lamp would have 7i)` volts of stress by the direct path butthe sense is wrong for conduction in the diode V2.

It will be noticed from Figure 7, that whilst the common even line 3|cannot exceed 80 volts it can fall below this value. Should it take anintermediate potential two neon lamps in series might strike. No. 2 andNo. 6 neon lamps have their free ends taken to G25-10) volts=115 voltsso that there is 57.5 volts across each neon lamp. Assuming No. 1 neonlamp has not struck, and that each neon burned at 45 volts there wouldbe an excess voltage of (115-90) =25 volts appearing across each of the100K resistors R23 in the series path. Thus the point A would fall 12.5volts on the nominal 125 to 112.5 volts and 102.5 volts would beavailable to strike No. 1 neon lamp. No. l neon lamp will fire, and whenit does so the stress on No. 2 and No. 4 neon lamps will reduce to 77.5volts, and since these cannot burn with less than 90 volts applied (45volts to each), they will be extinguished.

Thus, only one neon lamp-that selected by two high anodes in the 2nd,3rd and 4th switching circuits and the low anode in tac lst switchingcircuit, can light for any of the combinations of high and low voltageswhich might be applied to the neon lamps.

What we claim as our invention is:

l. A binary switching circuit comprising tivo electronic vacuum tubeseach having an anode, a cathode and a control grid, interconnectionsbetween the anode to cathode circuit of each tube and the control gridcircuit of the other tube adapted to permit the anode potential ofeither tube to exceed a predetermined upper value when the anodepotential of the other tube is restricted to less than a lowerpredetermined value, a source of anode voltage for said tubes, an inputconnection for a voltage pulse of predetermined sense and magnitude, thesense and magnitude of said pulse being so predetermined that said pulsecauses the anode potential of the tube hav ing an anode potentialexceeding the predetermined upper value to be reduced to less than thepredetermined lower value, a condenser series connected between theinput connection and each of said interconnections, a resistor seriesccnnected between the source of anode voltage and each of saidinterconnections, an inductance in shunt with each said resistor, saidinductance having relatively low direct current resistance but having inrespect of said pulse a high impedance relative to that of saidresistor, and a rectiier in shunt with said resistor and said inductanceadapted to offer a high impedance relative to that of said resistor inrespect to current duc to the negative fall of said pulse.

2. A binary switching circuit according' to claim 1 in which each tubehas at least one electrode more than an anode, a cathode and a controlgrid and in which said circuit is adapted to provide operatingconditions for the tubes such that either tube bottoms when its controlgrid voltage exceeds a predetermined value in a positive direction, andthe lower value of the anode potential of either tube is determined bybottoxning of that tube.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,306,386 Hollywood Dec. 29, 1942 2,416,158 Coykendall Feb.18, 1947 2,436,963 Grosdoff Mar. 2, 1948 2,503,662 Flowers Apr. 1l, 19502,513,442 Baker July 4, 1950 2,516,146 Brugh July 25, 1950 2,521,788Grosdoff Sept. l2, 1950 2,540,442 Grosdoit` Feb. 6, 1951 2,563,102Grossman et al. Aug. 7, 1951 2,563,123 Luck et al. Aug. 7, 1951

